1. Field of the Invention
The present invention relates to manipulation of bulk quantities of plate-like material. More particularly, the present invention relates to inverting a plurality of flat panel substrates and the like contained in a cassette-type carrier.
2. Description of the Invention Background
Many advances in the fields of electronics and optics, as well as other fields, have resulted from the development of multilayered plate-like devices, such as flat panel displays, optical devices, printed circuit boards and integrated circuits, that provide for more efficient performance of sequential operations. The performance characteristics of the multilayered devices are precisely determined by the composition, the dimensions and the surface characteristics of the individual layers. In the electronics industry, the desire for faster operating speeds and increased storage capacities have motivated advances in the miniaturization of components that has placed increasing demands on the capabilities of electronics manufacturers. In response to those demands, manufacturers have developed new techniques that are directed toward meeting the increased expectations of the industry.
One way to reduce the size of the device is to produce the devices having thinner layers thereby reducing the profile of the devices. A reduction in thickness can be realized especially for those layers in which the performance of the device is not dependent on the thickness of the layer, but merely depends on the presence of the layer, such as with insulating layers and the like. An equally strong motivation to produce thinner coating layers is that the coating materials, in many instances, are relatively expensive; therefore, a reduction in the thickness of the layers directly reduces the manufacturing cost of the device.
The most common method of coating the surface involves dipping the device into the coating material. This method, while providing for full coverage of the device does not produce consistent nor predictable coating thickness due to inherent slight variations in the coating properties and the numerous process conditions.
An alternative technique developed to produce coating layers that are thinner than normally achievable by dipping is to spin coat the material onto the substrate. In spin coating methods, a coating is applied to the surface of the substrate and then the substrate is spun at a sufficiently high rate, such that the rotational force developed by the spinning motion will cause the coating material to distribute over the surface of the substrate and a portion of the coating material will be spun off the substrate depending upon the strength of the rotational force. A problem with this method is that a significant amount of coating material is wasted, which increases overall production costs of the devices. In addition, the method may not provide a uniform thickness layer if the topography of the substrate is sufficiently rough, because the raised portions of the surface will tend to retain coating material that would otherwise have been spun off, thereby resulting in localized thickness variations.
A current preferred technique to produce thin coating layers 1 involves performing a meniscus coating of the surface, as is disclosed in U.S. Pat. Nos. 4,370,356 and 5,270,079 to Bok. In the meniscus coating process, the surface of a substrate to be coated is oriented downward and the substrate is moved relative to a coating applicator such that the bottom surface is brought into sufficiently close proximity to the coating applicator so as contact the coating material flowing from the applicator. The coating material forms a meniscus on the surface of the substrate that forms a uniform thin layer of the bottom surface as the substrate is moved relative to the applicator. The coating thickness is controlled by the distance between the applicator and the substrate surface and the flow of coating material through the applicator. This technique is capable of producing uniform coatings with thicknesses of less than a few microns. One inherent problem with this technique is that the surface to be treated must be facing downward, whereas in most other processes the surface to be treated must be facing upward and that typically the handling of the substrates is performed using the bottom of the substrate.
In addition, some multilayered devices, such as those used in optics, are produced by depositing layers on both sides of a substrate. Those types of devices presenting special handling and processing problems because handling of either side of the plate-like material can damage the layers affecting the performance of the device. The difficulty in handling those materials results in increased production costs due to the lower overall production yield. In addition, the increased complexity and amount of equipment and floor space required to produce those devices and the resulting longer processing times also serves to increase the production costs of devices that are coated on both sides.
Currently, plate-like materials that are to be treated using a meniscus coating technique or other techniques involving treating the material from below or on both sides can be handled in two ways. One method is disclosed in U.S. Pat. No. 3,610,397 issued Oct. 5, 1971 to Bok (the "'397 patent"). In the practice of the '397 patent, individual plates having first and second sides are transported along a conveyor belt with the second side of the plate in contact with the belt. The plates are passed in successive fashion through processing stages in which the first side of the plate is treated from above the plate. In order to perform the meniscus coating of the first surface from beneath the plate or to coat the second side of the plate, the plate is flipped over using a turnover device, so that the first side of the plate is in contact with the belt. Following either meniscus coating of the first side or coating of the second side, the plate must again be flipped over, using a turnover device or some other appropriate method to return the plate to its original orientation.
A problem with conveyor-type apparatuses and methods is that additional or more complex processors are required to handle the material. This results in increased equipment costs and floor space requirements. Also, the '397 patent recognizes the problems associated with individually turning over each plate twice in the prior art; however, the patent only discloses an apparatus intended to lessen the possibility of damage to the plate through the use of less complex machinery to perform the flipping operations. Another problem in the prior art, not addressed by the '397 patent, is when both sides of the material are to be treated, such as in optical applications, a freshly treated surfaces must be brought into contact with handling equipment. The contact with the handling equipment may damage the layers on the substrate and may also require increased processing times to allow the freshly treated surface to dry or harden prior to handling the surface.
A second method is disclosed in U.S. Pat. No. 5,368,645 issued Nov. 29, 1994 to Bok (the "'645 patent") involves the use of a vacuum chuck to invert the substrate or plate, thereby positioning the surface of the plate facing downward for treatment using the meniscus coating technique. In the '645 patent, a plate with the surface to be treated facing upward is positioned on a rotatable vacuum chuck. A vacuum is drawn through the chuck which holds the plate against the chunk and the chuck is rotated 180.degree. so that the surface to be treated is facing downward. The meniscus coating is applied to the plate and the vacuum chuck is returned to the upright position and the vacuum is released.
The use of vacuum chuck provides a solution to some of the problems associated with the prior art in that less additional equipment and floor space is necessary; however, the use of the vacuum chuck still requires the plates to be individually inverted, and a vacuum to be drawn and released, all of which increases the processing time and the potential for damage to the plates presented by individual handling of the material. Additionally, the vacuum chuck apparatuses, like the conveyor belt apparatuses, are not suitable for handling material that is to be treated on both sides.
The present invention is directed to providing a method and an apparatus for efficiently inverting plate-like material in bulk which overcomes, among others, the above-discussed problems so as to minimize the handling and processing equipment necessary to treat both sides of the plate-like material and to increase productivity and yield by minimizing damage to the material resulting from material handling operations.